Switching circuit employing transistor utilizing minority-carrier storage effect to mintain transistor conducting between input pulses



11, 1964 E. u. COHLER ETAL 3,144,563

SWITCHING CIRCUIT EMPLOYING TRANSISTORUTILIZING MINORITY-CARRIER STORAGEEFFECT TO MAINTAIN TRANSISTOR cououcmuc BETWEEN INPUT PULSES OriginalFiled Nov. 15, 1957 3 OPERATED DEVICE 25 H 12 g 24 E I7 ,5 2s 28INVENTORS EDMUND U. COHLER JOSEPH E. MONAHAN v BY ATTORNEY United StatesPatent Continuation of application Ser. No. 696,788, Nov. 15,

1957. This application Apr. 14, 1960, Ser. No. 23,124 6 Claims. (Cl.307-885) This invention is concerned with electronic switching circuits,and particularly with pulse-to-gate converters useful in electroniccomputers. The present application is a continuation of US. patentapplication Ser. No. 696,- 788, filed November 15, 1957, now abandoned.

Many computers, e.g., SEAC, DYSEAC, and UDOFT, employ dynamic circuittechniques in their logical realization, instead of the moreconventional bi-stable devices. In such circuitry, intelligence isconveyed by pulses occurring at fixed time intervals, with the presenceof pulses at a given interval indicating a 1 or a yes and the absence ofpulses, a 0 or no. The pulses are driven from an amplifier whichprovides logical operation upon and regeneration of an input pulse. Itis often desirable to have an external device driven from one of thesepulse amplifiers, and usually such devices have direct currentrequirements as distinguished from the pulse train output of theamplifier. In such instances, the computer is so programmed that thepulse amplifier from which the output is derived emits a string ofpulses for the time during which the output is desired. Some method isthen required for converting these pulse trains into an acceptablesignal for the output device. In a typical instance, the UDOFT computeris required to illuminate lights on panels, activate relays, and turn oncertain audio eifects.

In earlier designs for such pulse operation of direct current equipment,relays have been employed, because their low frequency response averagesthe output of the pulse amplifier tube, pulling up when the average output is high (a string of pulses is occurring) and dropping out when nopulses occur. This method, however, has disadvantages. From theviewpoint of economics and physical design, good relays are expensiveand bulky. Moreover, their electrical characteristics presentdifiiculties because of contact bounce and reactance in the relay coil;also, the relay must be placed in the primary of the pulse transformerto eliminate the loading effects of coil capacity and inductance. Thispresents a considerable problem when the pulse amplifier tubes areoperated near their cut-off voltage, as in some computers, because, dueto the variation in tube characteristics, certain tubes supply moreaverage direct current in the off condition than others do in the oncondition. As a result, the pull-up and drop-out characteristics of therelay must be individually adjusted for each operating tube and eachreplacement.

Accordingly, an object of the present invention is to provide a compact,economical, and reliable means for operating direct current devices inresponse to pulses of electric energy. Another, and related, object isto provide an improved, transistorized and pulse operated electronicswitch.

These objectives are attained, in accordance with the invention, byutilizing the operating pulses to produce saturating current in atransistor connecting the device to be operated to a direct currentsource, and through taking advantage of the minority storagecharacteristics of the saturated transistor to sustain transistorcurrent between pulses, thereby giving to a train of pulses a di-3,144,563 Patented Aug. 11, 1964 ice rect current switching capability.Other objects, features and advantages will be evident to those skilledin the art from the following description of a preferred embodiment ofthe invention and reference to the accompanying drawing, the singlefigure of which is a schematic diagram of a signal conversion andswitching circuit, according to the invention.

In the system shown by way of example in the drawing, operating pulsesderived from a pulse amplifier 27 are utilized to operate a directcurrent device 13 by applying them, through an input transformer 23, toa transistor 11 connected in series between the device to be operatedand a source of direct current potential. The values of the circuitparameters are so selected that the transistor is normally biased tocut-off, and is driven to saturation by the operating pulses.

The transistor 11 has its collector 12 connected, through a directcurrent load 13 (the device to be operated), to a terminal 14 to which asource of positive reference potential may be applied. Its emitter 15 isconnected, via terminal 16, to a source of negative reference potential,and its base 17, through resistor 18, to a junction point 19, which isin turn connected, via diode 20, to emitter 15. Another terminal 21 (towhich a voltage more negative than that at terminal 16 is applied) isconnected, through resistor 22 to the junction point 19.

The input to the circuit is derived from a transformer 23 having itssecondary winding 24 connected through an isolating diode 25 to junctionpoint 19 and its primary winding 26, to the pulse amplifier 27 Thedevice represented by load 13 may comprise direct current operatedequipment such as an indicator light, an analog instrument, a solenoidor relay, etc., which is operated in response to, and for a time periodcoincident with, a train of pulses from the amplifier 27.

If the transistor 11 has satisfactory saturability and storagecharacteristics (e.g., those of the 2N35A), the pulse train comingthrough transformer 23 and applied to base 17 will cause the transistor11 to act as a switch in the circuit between terminals 14 and 16 and thedevice 13 will be switched on and off in accordance with the presence orabsence of a train of pules at the input to the circuit. A more detailedexplanation of how this is accomplished follows.

With a positive 20 volts applied to terminal 14, a negative 3 voltsapplied to terminal 16, and a negative 20 volts applied to terminal 21,the transistor 11 is normally cut off; i.e., a minimum of current (lessthan 1 microampere) flows through the transistor and the load. Cut-offoccurs because the base 17 is biased slightly negative with respect tothe emitter 15 by positive current flow from the less negative potentialat terminal 16, through diode 20 and resistor 22 to the more negativepotential at terminal 21, with a slight voltage drop across the diode 20providing the required bias. (With the voltage ratings given, a value of560 ohms for resistor 22 will produce current adequate for thispurpose.)

The secondary winding 24 of transformer 23 is normally biased by anegative 4.5 volts applied to terminal 28, and the signal input to thecircuit from pulse amplifier 27 via transformer 23 comprises separatetrains of .4 microsecond pulses at .8 microsecond intervals vw'th anamplitude of approximately plus 3.5 volts. With this voltage input,resistor 18 is selected to have a value which will produce a basecurrent suflicient to saturate the transistor. (2.7K ohms has provedsatisfactory with the other values of circuit elements suggested.)

When the first pulse of a train is applied to base electrode 17 itdrives it positive with respect to emitter 15, and causes saturationcurrent to flow in the transistor. Saturation results in a concentrationof excess minority carriers (holes or electrons) at thecollector-emitter barrier; and at the end of the pulse, the excesscarriers drift to the collector region, thus sustaining collectorcurrent. Successive pulses enhance this effect to render the transistor11 continuously conductive. (For a discussion of transistor saturationand minority storage characteristics, sec Principles of TransistorCircuits, R. F. Shea, Wylie and Sons, 1953,11. 394.)

In its saturated condition the trainsistor offers a very low impedanceto the potential applied between terminals 14 and 16. Hence,substantially the full supply voltage and current capability of thetransistor is available to operate the load device 13 as a train ofpositive pulses is delivered through input transformer 23. When thepulse train is discontinued device 13 is turned off because thetransistor is biased to cut off by the voltage drop across iode 20.

The specific identities and values suggested for the elements of theillustrative circuit described merely reflect a workable combination,conventional substitutions may be made; and, with transistors currentlyavailable, supply potentials of forty volts or more are useable and loadcurrents of 100 milliamperes are reliably achievable.

It is to be understood that the invention is not limited specifically tothe embodiment illustrated and described, and is to be given the scopeof the following claims.

What is claimed is:

1. An electronic circuit which comprises a saturable transistor havingbase, collector and emitter electrodes and minority carrier storagecapability, means for applying a potential of one polarity through adirect current load to said collector, means for applying a potential ofopposite polarity to said emitter, means for applying a biasingpotential to said base, and means including a resistance for applying tosaid base a train of pulses of polarity, amplitude, current rating andrepetition rate adequate to drive said transistor to saturation andmaintain it in such saturated condition.

2. For operating a direct current device in response to control bypulsed voltage signals electronic signal converter which comprises: atransformer having primary and secondary windings; means for applyingpulsed voltage signals to the primary of said transformer and atransistor with base, collector and emitter electrodes; said transistorhaving its base connected through a resistor, a unidirectionalconducting device and the secondary winding of said transformer to abias potential, its collector connected through a direct current load toa source of poten tial of a given polarity, and its emitter connected toa source of potential of opposite polarity and, through a unidirectionalconducting device, to said base; and, a direct current device seriesconnected in the collector-emitter circuit of said transistor.

3. In an electronic circuit having as an input a train of positivevoltage pulses, means for applying direct current to a load in a mannerresponsive to and for a time duration substantially coincident with saidpulse train which comprises: a saturable transistor having minoritystorage capabilities and collector, emitter and base electrodes; meansfor connecting said collector through said load to a point of referencepotential of a given polarity; said minority storage capability of saidtransistor being so related to the frequency of the component pulses ofsaid pulse train that it maintains the transistor in saturation betweenthe recurring pulses of said trains; means for connecting said emitterto a point of reference potential of opposite polarity; impedance meansby which said pulse train input may be applied to said base, saidimpedance means having a resistance characteristic adequate to causepulses of said train to saturate said transistor; and, means forapplying to said base a potential of proper polarity and suflicientamplitude with respect to said emitter to render said transistorsubstantially non-conductive when said positive pulses are not appliedto said base.

4. In combination with an electronic circuit having an output ofsuccessive pulses of electric energy, means for operating a directcurrent device in response to said pulses which comprises: a saturabletransistor having minority storage capabilities and collector, base andemitter electrodes; said minority storage capability of said transistorbeing so related to the frequency of succession of said pulses that itmaintains the transistor in saturation between recurring pulses; asource of reference potential of a given polarity; a reference source ofopposite polarity; means for connecting said collector, through saiddirect current device, to said reference source of given polarity; meansfor connecting said emitter to said reference of opposite polarity; aunidirectional conducting device having some resistance to current flowin its forward direction connected between said emitter and said base,means for causing forward current to flow through said unidirectionaldevice thereby to bias said base with respect to said emitter as aresult of said some resistance; and impedance means connecting saidoutput to said base, said impedance means having a resistance adequateto cause said pulses to produce saturating current in said transistor.

5. An electronic circuit for operating a direct current device inresponse to successive pulses of electrical energy which comprises: asource of pulsed electric energy and a saturable transistor, havingminority storage capabilities and collector, emitter and baseelectrodes, said minority storage capability of said transistor being sorelated to the frequency of succession of said pulses that it maintainsthe transistor in saturation between recurring pulses, said collectorbeing connected through said device to be operated to a positive voltagesource, said emitter being connected to a first negative voltage source,said base being connected through a first resistor to a junction point,a first unidirectional conductor connected between said first negativeVoltage source and said junction point and polarized for positivecurrent flow in that direction, a second negative voltage source morenegative than said first source connected through a second resistor tosaid junction point, and means including a second unidirectionalconducting device connecting said pulse source to said junction point.

6. A system for operating a direct current operable device in responseto a train of pulses comprising in combination, a transformer havingprimary and secondary windings, means for applying voltage impulses tothe primary of said transformer, a transistor having base, collector andemitter electrodes, a unidirectional current conducting connectionbetween one terminal of the secondary of said transformer and the baseelectrode of said transistor, the other terminal of said secondary beingconnected to a source of bias potential, means including aunidirectional conducting device connected between the emitter and baseelectrodes for providing bias for said transistor, means for causingsaturation of said transistor upon application of a train of pulses fromthe secondary of said transformer whereby the minority storagecharacteristic of said transistor effectively generates a direct currentsignal at its collector, said direct current operable device beingconnected to said collector.

References Cited in the file of this patent UNITED STATES PATENTS2,899,571 Myers Aug. 11, 1959 2,913,597 Rowe Nov. 17, 1959 2,985,769Blount May 23, 1961 3,003,069 Clapper Oct. 3, 1961 FOREIGN PATENTS762,868 Great Britain Dec. 5, 1956 OTHER REFERENCES Transistor CircuitHandbook, by Garner, published by Coyne Electrical School, Chicago, Aug.1, 1956, pages 297401.

1. AN ELECTRONIC CIRCUIT WHICH COMPRISES A SATURABLE TRANSISTOR HAVINGBASE, COLLECTOR AND EMITTER ELECTRODES AND MINORITY CARRIER STORAGECAPABILITY, MEANS FOR APPLYING A POTENTIAL OF ONE POLARITY THROUGH ADIRECT CURRENT LOAD TO SAID COLLECTOR, MEANS FOR APPLYING A POTENTIAL OFOPPOSITE POLARITY TO SAID EMITTER, MEANS FOR APPLYING A BIASINGPOTENTIAL TO SAID BASE, AND MEANS INCLUDING A RESISTANCE FOR APPLYING TOSAID BASE A TRAIN OF PULSES OF POLARITY, AMPLITUDE, CURRENT RATING ANDREPETITION RATE ADEQUATE TO DRIVE SAID TRANSISTOR TO SATURATION ANDMAINTAIN IT IN SUCH SATURATED CONDITION.